This invention relates generally to improvements in dispensing pumps of the type set forth in U.S. Pat. No. 4,618,077.
Such manually actuated dispensing pump has a pump body capable of being mounted via a closure cap at the upper end of a container of liquid product to be dispensed, the pump body extending generally transversely above the closure cap. The pump body includes outwardly opening cylinder means providing at its inner end region a pump chamber for a manually reciprocable piston. The pump has a valve controlled inlet passage terminating in an inlet port opening into the pump chamber, and a valve controlled discharge passage leading from the pump chamber. The pump cylinder means has a vent port positioned outboard of the pump chamber and in open communication with the interior of the container. The piston has a pair of spaced, annular piston seals defining an annular chamber therebetween. The inboard seal operates in the pump chamber, and the outboard seal is positioned outwardly of the vent port in all operative positions of the pump piston. In a non-pumping position, the outboard seal seals off the vent port from the atmosphere. And, means such as vent rib is located at the wall of the cylinder means outboard of the vent port for interrupting the sealing function of the outboard seal during pumping to thereby open the vent.
However, it is difficult to prime such a pump since the air initially occupying the pump chamber is merely elastically compressed on the inward pressure stroke, without obtaining a sufficiently high pressure to move the discharge valve to an open position. On each suction stroke of the piston, the entrapped air merely expands, with the result that little or no liquid is drawn into the pump chamber. Thus a large number of pump strokes is required to prime the pump.